The objective of this study is to investigate the specificity of macrocyclic molecules for cations, by a combined X-Ray and thermodynamic study of cation complexes of synthetic cyclic polyethers and related compounds. The macrocycles chosen for study exhibit cation specificities similar to those found for many naturally occurring cyclic antibiotics, e.g., valinomycin and nonactin. The following studies are proposed for the year. (1) Study structures of uncomplexed and metal ion complexed cyclic polyethers and cyclic thiaethers to observe ligand conformational changes resulting from complexation. (2) Determine structures of unusual metal ion complexes of these ligands, for example complexes with stoichiometries other than 1 cation:1 ligand. (3) Continue to collect thermodynamic data for the complexation of metal ions with macrocyclic molecules to identify compounds which will be of interest in correlating thermodynamic and structural features.